The UN forecasts the Earth’s population will rise to 11 billion by 2100.

On social media on Thursday, Mr Sanders was likened to a Marvel movie villain.

Senator Ted Cruz said the Democratic White House hopeful’s remarks evoked Thanos, the Avengers baddie who kills half the world’s population with a snap of his fingers.

The Texas Republican joked: “In a surprise move, last night Bernie announced Thanos as his running mate.”

What did Bernie Sanders say?

The population control issue was raised during a CNN climate town hall event in New York City on Wednesday night during a question from an audience member.

Martha Readyoff, a teacher, said the topic was “poisonous for politicians, but it’s crucial to face”.

“Empowering women and educating everyone on the need to curb population growth seems a reasonable campaign to enact,” she began.

“Would you be courageous enough to discuss this issue and make it a key feature of a plan to address climate catastrophe?”

Mr Sanders, who is running for the Democratic presidential nomination to face President Donald Trump in the November 2020 election, replied: “Well, Martha, the answer is yes.

“The answer has everything to do with the fact that women in the United States of America, by the way, have a right to control their own bodies, and make reproductive decisions.

“The Mexico City Agreement which denies American aid to those organisations around the world that allow women to have abortions or even get involved in birth control to me is totally absurd.

“So I think, especially in poor countries around the world where women do not necessarily want to have large numbers of babies, and where they can have the opportunity through birth control to control the number of kids they have, is something I very, very strongly support.”

The Mexico City Policy is a US government rule to ban overseas organisations that receive federal aid from performing abortions, even with other funding.

The measure was rescinded under Democratic President Barack Obama, and reactivated in January 2017 by Mr Trump, a Republican.

What’s the other reaction?

US conservative outcry followed the Vermont senator’s comments.

Steve Scalise, a Louisiana Republican congressman, tweeted: “Bernie Sanders said YOUR tax dollars should be used to fund abortions in foreign countries to reduce population size.”